1. Field of the Invention
The present invention relates to a lens control device for use in a camera.
2. Related Background Art
Camcorders (VTR) have recently gained remarkable popularity, and, with their progress in the reduction of size and weight, the space and weight occupied by the lens and the automatic focusing device are rapidly decreasing.
In such a trend, the automatic focusing device is shifting from a so-called active type, having an emitter and a sensor for infrared light, to the passive type which detects the focus position from the image signal obtained from the image sensor.
On the other hand, in the lens unit, there is being widely introduced the lens of a so-called inner focus type, in which a lens for correcting the displacement of focal plane resulting from zooming also serves for focusing, and the front lens is fixed for achieving compactization.
FIG. 1 shows an example of such inner focus type lens, wherein shown are a first fixed lens group 101, a second lens group 102 for zooming, a diaphragm 103, a third fixed lens group 104, a fourth lens group 105 for correcting the displacement of focal plane resulting from zooming and for focusing (focusing lens or compensator lens), an image sensor 106, and an image receiving face 106a thereof.
FIG. 2 shows the relationship between the focal length or the position of the zooming lens 102 and the position of the focusing lens 105 for focusing to different object distances. When the focal length is fixed, namely when the zooming lens 102 is stopped, the focusing can be achieved by moving the focusing lens 105 parallel to the ordinate at a horizontal position corresponding to the focal length. Also in the course of a zooming operation, an image signal without image blur can be obtained by selecting a trajectory of the focusing lens 105 from FIG. 2 according to the object distance and driving the focusing lens 105 according to the trajectory and corresponding to the change in the focal length, thereby simultaneously effecting the correction on the displacement of focal plane resulting from zooming and the focusing.
FIG. 3 shows an example of the control method for the focusing lens 105 in the course of a zooming operation. The axes are same as those in FIG. 2. The trajectories shown in FIG. 2 are divided into plural areas according to the zooming lens position (focal length) and the focusing lens position (object distance), and the representative speed of the focusing lens is indicated in each area. Arrows with varying angles indicate the speed of the focusing lens 105.
In FIG. 3, the moving range (abscissa) of the zooming lens 102 is divided into 16 equal zones, and the lens driving speed is defined in each of the zones. The 16 equal zones will be hereinafter called zoom zones. By cutting the curves in FIG. 2 with the zoom zones, there are obtained sections of an almost same inclination in each zoom zone. In case the driving speed of the zooming lens, or the zooming speed, is constant, a same moving speed of the focusing lens 105 can be employed for different object distances if the speed or inclination of the focusing lens in each zoom zone is constant. Thus, each zoom zone is divided into portions of same inclination, as shown in FIG. 3, and a representative speed is given to each portion.
Thus, if the lens is focused at the start of zooming, it is rendered possible to cause the focusing lens 105 to follow the trajectory shown in FIG. 2 with an appropriate moving speed, by detecting the positions of the zooming lens and the focusing lens thereby determining the standard moving speed of the focusing lens, and applying correction by the focusing information from the auto focusing device.
However, let us consider a situation where the zooming lens 102 is positioned at the wide angle end in FIG. 2 and the focusing lens 105 is located close to a position P for focusing to an object at infinity. As will be apparent from FIG. 2, the cam trajectories for different object distances at the wide angle end are concentrated around a point A, and the focusing lens positions for infinity and for 3 meters may be contained in the depth of focus, depending on the lens and on the diaphragm aperture. Stated differently, in the zooming operation from the telephoto side to the wide angle side, the unfocused state does not occur easily because the trajectories are concentrated, but in the zooming operation from the side angle side to the telephoto side, a wrong trajector may be traced because the trajectories are divergent. In the passive automatic focusing device mentioned above, if the focusing lens position is contained in the depth of focus, the focusing lens may stop at a focusing position for an object distance of 3 m, even when an object at infinite position is being taken. If the zooming operation is conducted from the wide angle side to the telephoto side from a lens situation in which the lens is identified to be focused to an object distance different from the actual object distance, the lens continues to trace a wrong trajectory and eventually becomes positioned out of the depth of focus, thus generating a larger image blur, as the trajectories corresponding to different object distances diverge. Particularly, if the control of the automatic focusing device is interrupted and the zooming is conducted in the manual focusing state, defocus is almost certainly generated in the course of zooming, as the trajectory cannot be corrected during the zooming operation.